What is the proper anatomical term for the fetlock joint?
The metacarpophalangeal joint (MCP)
What is the anatomical name of the pastern joint?
The proximal interphalangeal joint (PIP)
What is the anatomical term for the coffin joint?
The distal interphalangeal joint (DIP).
What is the anatomical term for the coffin bone or the pedal bone?
What is the anatomical term for the navicular?
The distal sesamoid bone.
What is the anatomical name for the cannon bone?
What are the anatomical terms for the splint bones?
Metacarpal 2 & Metacarpal 4
Of all the extrinsic muscles, which is the most important in terms of passive stay and why?
The serratus ventralis on the medial aspect of the scapula helps to attach the trunk of the horse to the limb in a sling. It is a large muscle in the horse, mixed with collagenous tissue that is actually tendinous tissue that suspends the weight of the horse from the scapulae.
What is meant by passive stay? What are the mechanisms involved in passive stay?
The stay apparatus refers to arrangements of tendons and ligamentous tissue in the fore- and hindlimbs that support the joints of the animal by storage of energy and weight transfer to reduce muscle fatigue. These mechanisms of the stay apparatus enable the animal to rest while standing up, as in the case of the horse.
Serratus ventralis "sling"
Biceps brachii tendon- Lacertus fibrosus - Extensor carpi radialis
Check apparatus (two check ligaments)
Suspensory apparatus - suspensory ligament
Reciprocal apparatus - peroneus tertius & superficial digital flexor tendon
Check apparatus (one check ligament, below the tarsal joint [hock])
Suspensory apparatus - suspensory ligament
Passive Stay Apparatus of the Forelimb
Explain how the serratus ventralis muscle plays a role.
The serratus ventralis muscle originates at the first to seventh ribs & the transverse processes of the cervical vertabrae. It inserts on the medial aspect of the scapulae.
When the body is at rest, the thick tendinous layer of this muscle, called the dorsoscapular ligament, attached to the thorax, supports the weight of the trunk, as a "sling".
While this doroscapular ligament is able to support the weight, it pulls down from the spine of the scapula. The gravitational pull would cause the joints of the forelimb to FLEX, and the animal to fall to the ground, were it not for the other passive-stay mechanisms:
Biceps brachii tendon
Explain how the biceps brachii tendon contributes to the passive stay apparatus.
The biceps brachii muscle, which originates at the superglenoid tubercle of the scapula, has a collagenous tendon running through it that, when weight is placed on the forelimb, tenses up. This tensing of the muscle and tendon has the effect of extending the shoulder joint.
The biceps brachii tendon's "short tendon", a thick tendon, inserts on the radial tuberosity, while the "long tendon", also known as the lacertus fibrosus, continues distally to connect the biceps brachii tendon to the tendon of the extensor carpi radialis muscle. The long tendon inserts at the proximal end of the metacarpus.
Thus the tension in the biceps brachii tendon is transmitted down across the front of the forelimb, keeping the shoulder extended. The elbow ligaments keep the elbow joint from collapsing.
What is the check apparatus of the forelimb?
Two check ligaments:
Superior check ( Accessory ligament to superficial digital flexor tendon) - at the level of carpal joint, a thick ligament from the caudal distal radius combines with the tendon of the superficial digital flexor muscle. This "check" prevents over-extension of the carpal joint and metacarpophalangeal joint ( fetlock joint).
Inferior check (Accessory ligament to the deep digital flexor tendon) - at the level of about mid-metacarpal 3, a thick ligament from the palmar carpal ligament combines with the deep digital flexor tendon. This "check" prevents over-extension of the fetlock, pastern and coffin joints.
Why is the intertubercular groove of the humerus so important to the stay apparatus of the forelimb?
It is a deep groove in which the biceps brachii tendon runs. It acts like a track for the tendon. When weight is placed on the forelimb, the muscle and tendon push cranially into the track, bracing the shoulder into extended position.
What are the components of the Suspensory Apparatus in the equine forelimb?
There are five components:
1. The interosseus aka suspensory ligament - most important
2. Common digital extensor muscle (cranial)
3. Collateral sesamoidean ligaments
4. Palmar (intersesamoidean) ligaments
5. Distal sesamoidean ligaments
How does the suspensory apparatus contribute to passive stay?
Suspensory or interosseus ligament - Proximally this attaches to the distal row of carpal bones and metacarpus. It runs distally on the palmar surface of the metacarpal bone and then bifurcates proximal to the fetlock, with a small branch connecting to each sesamoid.
It then continues distally on the dorsal surface of the proximal phalanx and joins the common digital extensor, which continues to insert on the middle and distal phalanges.
Collateral sesamoid ligaments – connect the abaxial surface of the sesamoid to the metacarpus and proximal phalanx.
Palmar ligaments – connect the sesamoid bones together.
Distal sesamoidean ligaments – connect the sesamoids to the proximal and middle phalanges.
These ligaments can be seen as a direct continuation of the interosseous muscle with the sesamoids emdedded in it.
Thus by virtue of the ligamentous arrangement, the suspensory apparatus without any muscular action can resist extension of the fetlock and pastern through the distal sesamoidean ligaments and resist flexion of the pastern and coffin joints through the common digital extensor tendon.